Assuming you have both frameworks installed (and rebooted the machine afterwards),
then from the IIS Manager you would open the Properties dialog for the IIS "component" (i.e. "level")
you wish to switch. When I say "component" or "level" I'm referring to the
fact that inside the IIS manager, it is possible to open the Properties dialog
for 3 different "levels".

For ALL websites... the node labeled "Web Sites" (whis is global for all sites)

The node for an indididual website

Or the properties for an individual Virtual Directory (VD) within a specific website

How you installed BD .NET (global in the GAC, for a specific website, or for a specific VD)
will decide for which "level" you should open the Properties Dialog inside the IIS manager.

Once you have the properties dialog up, go to the tab labeled "ASP.NET" and
select (from the drop-down list) the .NET version you want that "level" to use.
Apply the change.
All of this so far is just .NET "stuff" and really has nothing to do with BlueDragon.

If using BlueDragon .NET 6.2.1 then you could immediately go to the tab named "Home Directory" (or "Virtual Directory" if your "level" is a VD) on that same Properties dialog, and click the "Configuration..." button and verify that your .cfm, .cfml, and .cfc mappings there are pointing to the correct version of the .NET framework.
Note that BD 7.0 will not support .NET 1.1, it will only support .NET 2.0. So BD 7.0 won't support this sort of framework version "switching".
Also note that if you installed .NET 2.0 *after* BD .NET 6.2.1 was already installed, then certain BD-related components needed to support .NET 2.0 were not installed (since .NET 2.0 was not present at BD install time).
So you'd need to uninstall BD and then reinstall it with .NET 2.0 being present on the machine before BD could run on .NET 2.0.